In a double-fired heater, at least one heat exchange tube, which carries a process fluid (liquid or gas), is heated by combustion from two opposing sides of the tube in a radiant section of the heater. This invention relates to a subclass of such heaters, which will be referred to as "horizontal tube heaters," in which the tube (or tubes) winds back and forth in horizontal lengths to form a coil panel (or panels). The coil panel is supported within the radiant section by tube supports. Horizontal tube heaters are used in such processes as "cracking" ethylene dichloride (EDC) into vinyl chloride for use as fibers and plastics (such a heater is referred to as an EDC furnace), vaporizing sulfur in petrochemical applications, heating coking feedstock, and the like. One example of a horizontal tube heater, used for heating coking feedstock, is illustrated in U.S. Pat. No. 5,078,857, to Melton.
As a practical matter, most horizontal tube heaters will contain a convection section in addition to the radiant section. In the convection section, which is employed downstream and at a higher elevation than the radiant section, a convective tube coil (or coils) is exposed to a flow of hot exhaust from combustion in the radiant section.
In many horizontal tube heater applications, such as those mentioned above, the tube and tube supports are subjected to harsh operating or environmental conditions. These conditions can lead to significant corrosion, and wear and tear on the tube and supports, requiring the tube and/or supports to be periodically replaced--typically after five to ten years of service. In a typical horizontal tube heater, the replacement of the tube and/or supports is an onerous task.
For example, U.S. Pat. No. 3,384,053, to Fleischer, teaches a doublefired heater with an offset chimney. A tube coil is top-supported by hinged supports, which are suspended from the heater structural framework and extend into the heater through small openings through the heater roof. The openings are preferably closed around the support with cement. The heater taught by Fleischer appears to suffer from the same tube-replacement drawbacks as most horizontal tube heaters. Traditionally, the horizontal lengths of tube have to be cut into sections and removed longitudinally, one section at a time, through a door in a furnace end wall. The sectioning, lowering and removal of tube lengths located at higher elevations in the heater can be difficult and somewhat hazardous. Also, the replacement tube has to be inserted into and assembled inside the furnace in a similar manner. Further, because it has not been practical to replace the tube supports without dismantling the tube or cutting apart the tube support, the task is still onerous even if only a tube support needs replacement.
Attempts have been made to provide a removable end wall through which the entire tube coil panel can be removed on slides or rails. These attempts have generally proven to be costly and impractical. One such attempt is illustrated in U.S. Pat. No. 2,456,787, to Kniel. This patent illustrates a heater, designed not to employ a convection section, in which one tube coil is double-fired and two peripheral coils are single-fired (i.e., exposed to flame on one side only) in a furnace chamber. A pair of exhaust ducts extend from the furnace chamber roof. The double-fired tube coil is supported in the chamber by coil supports, through which horizontal lengths of the tube coil extend. The coil supports suspend from a longitudinal track (located above the furnace chamber between the exhaust ducts) down through a slot (parallel to the track) in the furnace chamber roof and into the furnace chamber. The roof slot is normally closed around the supports by hinged closures, the inner surfaces of which are formed of refractory material. Another slot, which is also normally closed by a hinged closure with a refractory inner surface, is provided in the end wall. When the roof slot and end-wall slot closures are opened, the coil can be removed or inserted through the end-wall slot by moving the support along the track. This is a complex arrangement, requiring large openable closures in both the roof and end wall, as well as structure extending well past the furnace chamber end wall to support the track that carries the coil as it is removed through the end wall. Further, no provision is made for interchangeability of the tube supports independently of the tube coil.
Mention should be made of another class of double-fired heaters, referred to herein as "vertical tube heaters," which utilize tubes arranged in vertical lengths instead of horizontal. The construction features, applications and maintenance needs of vertical tube heaters are quite different from horizontal tube heaters, and, therefore, much of the discussion herein will not apply to vertical tube heaters. For example, in most vertical tube heaters, the vertical tube lengths are supported individually from outside the radiant section by a system of linkages and counterweights. Generally, no support members are employed within the radiant section of the heater. As with horizontal tube heaters, however, the vertical lengths are typically longitudinally inserted and removed. Due to the orientation of the tube lengths, they are typically inserted and removed through small openings provided in the roof of the radiant section. Some examples are illustrated in U.S. Pat. Nos. 3,230,052 and 3,265,043, both to Lee, et al.; 3,348,923, to Demarest; and 4,955,323, to Ziemianek. No provision is made in any of these patents for insertion and removal of multiple tube lengths as a unit. Obviously, with no in-radiant-section tube support, there is also no provision for interchanging such a support independently of the tube.
Accordingly, there is a need in the art for a horizontal tube heater in which provision is made for simplified removal and replacement of a worn tube coil panel.
There is a further need in the art for a horizontal tube heater in which the coil panel can be removed as a unit, and a replacement coil panel can similarly be inserted as a unit.
There is a still further need for a horizontal tube heater in which a tube support can be removed and replaced independently of the coil panel itself.